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A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices

Periodic Reporting for period 1 - HOCOM (A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices)

Reporting period: 2019-09-01 to 2021-03-31

Optoelectronic devices such as solar cells and light emitting diodes (LEDs) are key to a sustainable energy future. They have two fundamental building blocks. First, a photoactive material to transform light into electrical current or vice versa. Second, contact materials to collect (or inject) this current. Because light has to reach the photo-active material from outside (as in solar cells) or it has to exit the device and reach the environment (as in LEDs) one of the two contacts has to be transparent. There is a long-standing problem in the science and technology of transparent contacts. Specifically, we are only able to produce transparent contacts where electrons carry the current. However, there is a second type of current flowing in the opposite direction in the photo-active material: a hole current. This current is carried by electron vacancies (“holes”), which are in some way analogous to bubbles in water. The performance level of transparent hole conductors is, however, much lower than transparent electron conductors. Using a simple performance figure of merit, this means in practice that the product of their electrical conductivity and their optical transmission is not as high as it should be for successful application in real optoelectronic devices.

This issue severely limits our design options for solar cells and LEDs, it prevents the realization of transparent electronics, and it is a symptom of a gap in our scientific understanding of materials. Advances in hole transparent conductors would likely lead to improvement across all areas of optoelectronics (a key field for renewable energy and energy efficiency). Answering the scientific question “what makes a good transparent hole conductor?” would also be likely to trigger new fields and opportunities in materials science.

The main goal of HOCOM is to experimentally evaluate certain phosphide materials as potential transparent hole conductors. Synthesis of these candidate materials in thin-film form (as relevant for optoelectronic devices) is made possible by a unique deposition setup at the National Renewable Energy Laboratory (NREL, USA) dedicated to phosphides. Detailed characterization of the most interesting phosphides will be performed at the Helmholtz Zentrum Berlin (HZB, Germany).
In the initial three-month secondment at HZB, we investigated the moisture-dependent electrical properties of CuI, which had emerged as a particularly promising transparent hole conductor shortly before the beginning of HOCOM. Surprisingly, we found that the conductivity of CuI increases upon water adsorption. We published detailed humidity-dependent electrical characterization in a journal article and proposed possible mechanisms for the conductivity improvement.

In the 16-month outgoing phase at NREL, we applied combinatorial research techniques to rapidly evaluate the potential of selected phosphide thin films as transparent hole conductors. The growth chamber was a unique sputter system equipped with diluted phosphine, which allowed us to grow phosphide films by reactive sputtering of metallic targets. The two main materials we investigated were BP (boron phosphide) and CaCuP. For the case of BP, we were able to obtain the correct stoichiometric ratio between B and P, but all the grown BP film were amorphous and had a very low conductivity. Post-annealing these films allowed us to obtain the desired BP crystal structure and more encouraging conductivities. Under specific growth and annealing conditions, the BP films exhibited hole conduction as desired and were at least partially transparent. However, the performance figure of merit was rather low. As a second track, we investigated reactive sputtering of CaCuP. This compound could be indeed synthesized in one step at an appropriate temperature. We found very high hole conductivity under specific process conditions. The optical transparency was less than optimal, in disagreement with theoretical expectations. We are currently working with computational collaborators to understand this discrepancy. Nevertheless, CaCuP still exhibits a reasonably high figure of merit and a very remarkably high conductivity for a p-type semiconductor without external doping. Thus, our results warrant further investigation of this exotic compound.
We have carried out the first explicit experimental investigation of BP and CaCuP as transparent hole conductors. CaCuP had never been reported in thin-film form, and BP had never been synthesized by reactive sputtering. We were able to grow both compounds as single phases and demonstrate encouraging performance towards our targeted application. We expect that this early-stage demonstration will fuel further research in reactive sputtering of phosphides, which could be a versatile method to rapidly explore the highly uncharted field of phosphide thin films. Our results also confirm the need to go beyond oxide materials when searching for new potential transparent hole conductors. In general, HOCOM is convincing us even more of the need to rapidly explore new materials which could exceed the performance of existing materials. This is particularly important in a society which is more and more dependent on high-performance materials to achieve its urgently needed renewable energy goals.
In the second part of HOCOM, we plan to further characterize BP and CaCuP using the advanced characterization setups available at HZB. In particular, we will try to identify the dominant defects responsible for hole conduction by various spectroscopy techniques. We will also investigate the band alignment of BP and CaCuP with common semiconductors, to evaluate their applicability in a real device. In case of a positive answer, we will fabricate a simple diode device to show that phosphide hole conductors are a feasible option for next-generation optoelectronic devices.
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